Recently, researchers have begun to use long period gratings (LPGs) to measure refractive index changes. The principle of operation of a LPG is based on the phase-matching condition between the guided and cladding modes in an optical waveguide. Light launched in the waveguide core interacts with the LPG, a periodic refractive index modulation in the fiber core, and is converted into a number of modes contained within the waveguide cladding. The light that is coupled from the core to the cladding is spectrally dependent upon the fiber, grating parameters, and the surrounding environment. As the refractive index changes around the optical fiber, the coupling wavelength changes and a spectral shift can be observed to determine the refractive index.
Observation of the coupling wavelength spectral location is generally performed using static spectral filters, dynamic spectral filters, or optical spectrum analyzers to simultaneously observe the entire spectrum. In each case, enhanced sensitivity is achieved by the measurement system if the LPG coupling wavelength has a greater response to refractive index. Furthermore, increased performance of the LPG sensing element can lead to simplified electronics that do not require high resolution components.
In LPG sensors, the coupling wavelength is dependent upon the refractive index of the fundamental mode, the periodicity of the LPG sensing element, and the effective index of the coupled mode. The effective index is determined by the refractive indices of all the materials through which the mode propagates. For example, in an optical fiber, light contained within a cladding mode propagates with most of the energy contained inside the fiber. However, an evanescent field exists which decays exponentially outside the fiber. The effective index of this mode is approximately the ratio of the two refractive indices and is relative to the energy distribution of the mode. Previously, LPG sensitivity to refractive index was increased by coupling into higher-order modes. These modes have a larger evanescent field outside the fiber so changes in the external refractive index have a greater effect on the effective index of the coupled cladding mode. As a result, a larger wavelength shift was obtained.
However, coupling into higher-order modes has limitations. First, coupling to higher order modes depends on the modal overlap between the fundamental mode and the cladding mode. Each mode has an energy distribution and for modal coupling to occur, there must be an overlap of this energy distribution. For higher order modes, this overlap with the fundamental mode is reduced. Therefore, to couple light into these modes better coupling conditions are required which are sometimes difficult to obtain. Furthermore, coupling into higher-order modes does not necessarily increase the sensitivity.
An object of the present invention is to enhance the sensitivity of an optical fiber having at least one long period grating by balancing the physical parameters in the optical fiber.
Another object of the invention is to provide a process for preparing an optical fiber having at least one long period grating with enhanced sensitivity by balancing the propagation characteristics of the fundamental and coupled modes.
Another object of the invention is to provide a process for measuring an environmental perturbation using an optical fiber having at least one long period grating with enhanced sensitivity.